A known method for laser beam processing of thin film solar cells is disclosed in WO 03/061 013 A1. A known method for laser beam processing for trimming a resistance, a reactance or a capacitance of a circuit board after each electronic component is mounted to the circuit board is disclosed in U.S. Pat. No. 5,670,068.
In the known method for laser beam processing mentioned above, a laser beam is deflected by scanning mirrors and focused onto an element to be processed. Due to temperature and long term drift of the scanning mirrors the lateral precision is limited. The precision is further reduced, if large area substrates are used.
In case a to-be-processed element has a low mechanical stability, like foils, it is even more difficult to exactly establish the position of the laser beam relative to structures from an earlier processing step.
To avoid such problems, a camera with suitable optics is used to determine the exact position of the laser beam. Since for high precision laser scribing, the field of view is limited for the required large magnification, either a plurality of cameras is necessary for a large area, or the field of view of the camera has to be scanned over the processing area, preferably by coupling with the scanning optic of the laser beam delivery. In the latter case, a dichotic beam splitter or a switch-able mirror is placed into the path of the laser beam to separate the laser beam from the observation light followed by optical components to compensate the chromatic error of the focusing optic, if different wavelengths are used.
Therefore, a to-be-processed element having a low contrast, i.e. an element comprising layers with a light transmission ratio (total transmittance) below 10% as they can be found in thin film solar cells, may not be suitable for image processing using the known methods for the following reasons.
Due to the fact that the angle of reflection changes during scanning, if illumination is not completely diffuse, the intensity of the reflected light from the illumination reaching the imaging optic changes considerably. Therefore, a homogeneously illuminated image suitable for machine vision is hard to achieve. Especially in the case of a to-be-processed element with a light transmission below 10%, reflections from the top surface may be more intense than from structures in one or several underlying layers. The contrast of the image may not be suitable for image recognition in this case.
In summary, it is a difficult task to precisely structure layers on substrates with low transmission and dimensional stability with the known teaching.
It is therefore one object of the present invention to overcome the above-mentioned difficulties and disadvantages of the known methods and apparatus for laser beam processing.
This and other objects are reached by the features given in claim 1. Further embodiments as well as an apparatus are defined in further claims.
The present invention is directed to a method for laser beam processing of an element, be it flat or curved, that has a total transmittance for light of at least 10−5. The method comprises the steps of:                processing the to-be--processed element on one side by a laser beam;        illuminating the to-be-processed element on the other side by light of a illumination unit, the light having at least one predetermined wavelength;        recording residual light on the one side, which residual light results from a light transmission through the to-be-processed element by light of the illuminating unit;        determining laser beam processing position by analyzing the recorded residual light; and        adjusting the laser beam processing position.        
An embodiment of the present invention is further characterized by                providing at least one alignment mark on the to-be-processed element; and        determining laser beam processing position relative to the alignment mark.        
The alignment mark is clearly visible in the acquired image because the contrast is significantly increased due to the positioning of the illumination unit on the other side of the element to be processed with respect to the recording unit for the residual light. As a result thereof, the position of the laser beam or the position of the to-be-processed element in relation to the laser beam can be calculated with high precision by image processing.
Another embodiment is characterized by                providing alignment marks on the to-be-processed element;        monitoring position of the alignment mark; and        adjusting laser beam processing relative to the position of the alignment mark.        
A further embodiment is characterized by further comprising the step of focusing the laser beam at a predefined layer of the to-be-processed element.
A further embodiment is characterized by using a scanning unit for adjusting the laser beam processing position.
A further embodiment is characterized by using mounting means for adjusting the laser beam processing position.
The mounting means is placed, for example, in-between the illumination unit and the focusing optic such that the light is transmitting the alignment mark and the to-be-processed element and is collected by the imaging optic on the side of the laser unit.
A further embodiment is characterized by                providing conditioning means for conditioning the laser beam before it is used for laser beam processing, and        using at least some of the conditioning means for treating the residual light before the step of recording the residual light.        
A further embodiment is characterized in that the to-be-processed element is flexible and in particular comprises a substrate and at least one layer to be processed.
Furthermore, an apparatus for laser beam processing of an element that has a total transmittance for light of at least 10−5 is disclosed. The inventive apparatus comprises:                a laser unit for generating a laser beam on one side of the to-be-processed element;        an illumination unit;        an imaging system comprising a sensor unit on the one side of the to-be-processed element, the sensor unit recording residual light that results from light of the illumination unit;        a scanning unit for adjusting the laser beam processing position; and        a control unit,wherein the control unit is operatively connected to the laser unit, the imaging system and the scanning unit, characterized in that the illumination unit is positioned on the other side of the to-be-processed element in relation to the laser unit.        
The present invention particularly relates to an apparatus for high precision laser beam processing of multilayered thin films on substrates, e.g. polymer or glas with a total transmittance for light as low as 10−5 at the illumination wavelength, for example, organic light emitting diodes and, more particularly, thin film solar cells of Si and compound semiconductors , especially based on ternary chalcogenide semiconductors, which consist of an element or a combination of elements from group 11 of the periodic table of elements, an element or a combination of elements from group 13, and an element or a combination of elements from group 16 (chalcogen group) in any ratio.
More particularly, a ternary chalcopyrite compound with composition (CuxAg1-x)1(InuGavAlw)1(SeyS1-y)2 may be used, where 0≦x≦1, 0≦y≦1, u+v+w=1. It may be advantageous to have some deviations from the exact 1:1:2 stoichiometry. For example a slightly Cu deficient composition of Cu0.9(In0.75Ga0.25)1.1Se2 may be used or compositionally graded single or multilayers.
An embodiment of the invention is characterized in that the to-be-processed element comprises at least one alignment mark.
A further embodiment of the invention is characterized by further comprising a focusing optic for the laser beam, the focusing optic being positioned in-between the scanning unit and the element to be processed.
A further embodiment of the invention is characterized in that the focusing optic is positioned in-between the laser unit and the scanning unit.
A further embodiment of the invention is characterized by further comprising mounting means for further adjusting the laser beam processing position.
A further embodiment of the invention is characterized in that the to-be-processed element is flexible and in particular comprises a substrate and at least one layer to be processed.
A further embodiment of the invention is characterized in that the residual light and the laser beam are conditionable by at least the focusing optic and/or the scanning unit.
A further embodiment of the invention is characterized by further comprising a separation unit for separating the residual light from the laser beam.
A further embodiment of the invention is characterized by further comprising optical elements to correct chromatic aberrations of the focusing optic.
A further embodiment of the invention is characterized in that the to-be-processed element (12) comprises at least one layer having at least one of the following characteristics:                a metallic or semi-metallic conductivity used as electrical contact in solar cells;        a semiconductor used as absorber layer in solar cells;        a ternary chalcopyrite semiconductor used as absorber layer in solar cells with composition (CuxAg1-x)1(InuGavAlw)1(SeyS1-y)2 with 0≦x≦1, 0≦y≦1, u+v+w=1;        a transparent electrically conductive material used as electrical contact in solar cells.        
A further embodiment of the invention is characterized in that the to-be-processed element comprises a substrate consisting of at least one of the following:                polymer;        polyimide;        glass.        
A further embodiment of the invention is characterized in that the illumination unit emits at least one wavelength larger than 800 nm.
A further embodiment of the invention is characterized in that the imaging system comprises the same focusing optic used to focus the laser beam, which is an F-Theta lens color corrected for 532 nm and 1064 nm, and a dichotic beam splitter to separate both wavelength, wherein the laser unit emits a wavelength of 532 nm, and wherein the illumination unit emits at least light at a wavelength of 1064 nm.
A further embodiment of the invention is characterized in that the illumination unit emits a laser light having a wavelength of 1064 nm, and the laser unit uses a part of this light to generate 532 nm laser light for laser beam processing.
A further embodiment of the invention is characterized in that the mounting means comprise a vacuum chuck made from a translucent porous material with transmission at a wavelength of the light of the illumination unit.
To-be-processed elements with low dimensional stability, like foils, require a rigid support to keep the layer to be processed within the focal depth (Rayleigh length) of the laser beam, which is small, if a small size of laser focus is required. Therefore, a mounting means, like an electrostatic or vacuum chuck, is required, which is usually opaque. Illumination comprising one or a plurality of lamps, LEDs (Light Emitting Diodes) or lasers is therefore placed adjacent to the camera such that the light is reflected from the to-be-processed element into the imaging optic, either directly or diffusely.
A further embodiment of the invention is characterized in that the mounting means comprise at least two rotate-able cylinder shaped rolls arranged parallel to each other, where the flexible element is positioned in a tangent plane of said rolls, which lies in a focal plane of the focusing optic, and the illumination unit is placed such that the light of the illumination unit is passing between the rolls.
A further embodiment of the invention is characterized in that the mounting means comprise a convexly shaped surface with transmission at a wavelength of light of the illumination unit, where the flexible element is positioned at the surface, which lies within a focal area of the focusing optic.
A further embodiment of the invention is characterized in that the mounting means comprise a rotate-able cylinder-shaped roll with transmission at a wavelength of light of the illumination unit, where the flexible element is positioned along a part of a circumference of the roll, which lies within a focal area of the focusing optic.
The invention will now be described in more detail by reference to drawings showing exemplified embodiments.